1. Technical Field
The present application relates to the measurement of objects with respect to their position and dimension, in particular, to geodetic measurements. The invention relates, in particular, to a geodetic target for use together with a geodetic measurement instrument for performing geodetic measurements.
2. Brief Description of the Prior Art
In geodetic measurement instruments, measurement is frequently performed with respect to a geodetic target disposed at a distance from the measurement instrument. Examples of geodetic measurement instruments are theodolites, tachymeters (total stations) (particularly video tachymeters), leveling instruments (levelers) and laser scanners. The present application is, however, not limited to such kinds of measurement instruments. The geodetic target may be a natural target, such as a landmark (for example a soil formation, building, tree, etc.) or an artificial target (for example a surveying or leveling rod, a reflector etc.). Locating the geodetic target may be performed manually by a user or automatically by the measurement instrument. An automatic locating and, if required, tracing of the geodetic target is also referred to as “tracking”. An automatic locating and tracing of a geodetic target is, in particular, performed if the geodetic target is not stationary and thus mobile (for example a vehicle used on a construction site or in a mine with or without artificial geodetic target mounted thereto, a surveying rod supported by a user, etc.).
As far as artificial geodetic targets are concerned, a distinction can be made between so-called passive and so-called active geodetic targets.
In the case of passive geodetic targets, either the measurement instrument detects an image of the geodetic target itself, or the measurement instrument emits radiation that is reflected by the passive geodetic target and detected by the measurement instrument. Examples of passive geodetic targets are surveying and leveling rods, as well as reflectors (particularly retroreflectors). Examples of reflectors are prisms, retroreflectors or triple prisms. An example of a geodetic measurement instrument making use of a passive geodetic target is disclosed in WO 2008/014813 A1.
In order to render passive geodetic targets distinguishable from one another U.S. Pat. No. 6,384,907 proposes that each geodetic target comprises a sub target for measuring purposes and plural sub targets for identification purpose. The sub targets have different colors for identification purposes and a fixed spatial relation to the sub target for measurement purposes.
In case of active geodetic targets, radiation is emitted by the geodetic target itself, which is received by the measurement instrument. The radiation emitted by the active geodetic target may have a pattern (for example modulation of frequency or amplitude, embedding of analog or digital information, etc.) that renders the radiation emitted by the active geodetic target distinguishable from other radiation of the same kind. An example of an active geodetic target that can be used in connection with a measurement instrument is disclosed in WO 2008/145158 A1.
U.S. Pat. No. 7,184,151 B1 proposes to provide a geodetic target with plural active or passive sub targets that differ from one another in intensity and/or size. This renders plural geodetic targets distinguishable from each other.
From WO 92/16817 it is known to selectively activate an active geodetic target from outside. A sequential activation thus allows distinguishing between several active geodetic targets.
It has been found that geodetic targets can still be further improved in order to be reliably detectable by a measurement instrument detecting the geodetic target and to be distinguishable by the measurement instrument from other geodetic targets.